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  <section id="id1">
<h1>序列化（编码、解码）<a class="headerlink" href="#id1" title="Permalink to this heading"></a></h1>
<p>When a structure is sent over the network or written to disk, it is
encoded into a string of bytes.  Serializable structures have
<code class="docutils literal notranslate"><span class="pre">encode</span></code> and <code class="docutils literal notranslate"><span class="pre">decode</span></code> methods that write and read from <code class="docutils literal notranslate"><span class="pre">bufferlist</span></code>
objects representing byte strings.</p>
<section id="adding-a-field-to-a-structure">
<h2>Adding a field to a structure<a class="headerlink" href="#adding-a-field-to-a-structure" title="Permalink to this heading"></a></h2>
<p>You can see examples of this all over the Ceph code, but here’s an
example:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">AcmeClass</span>
<span class="p">{</span>
<span class="w">    </span><span class="kt">int</span><span class="w"> </span><span class="n">member1</span><span class="p">;</span>
<span class="w">    </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">member2</span><span class="p">;</span>

<span class="w">    </span><span class="kt">void</span><span class="w"> </span><span class="nf">encode</span><span class="p">(</span><span class="n">bufferlist</span><span class="w"> </span><span class="o">&amp;</span><span class="n">bl</span><span class="p">)</span>
<span class="w">    </span><span class="p">{</span>
<span class="w">        </span><span class="n">ENCODE_START</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">encode</span><span class="p">(</span><span class="n">member1</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">encode</span><span class="p">(</span><span class="n">member2</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="n">ENCODE_FINISH</span><span class="p">(</span><span class="n">bl</span><span class="p">);</span>
<span class="w">    </span><span class="p">}</span>

<span class="w">    </span><span class="kt">void</span><span class="w"> </span><span class="nf">decode</span><span class="p">(</span><span class="n">bufferlist</span><span class="o">::</span><span class="n">iterator</span><span class="w"> </span><span class="o">&amp;</span><span class="n">bl</span><span class="p">)</span>
<span class="w">    </span><span class="p">{</span>
<span class="w">        </span><span class="n">DECODE_START</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">decode</span><span class="p">(</span><span class="n">member1</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">decode</span><span class="p">(</span><span class="n">member2</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="n">DECODE_FINISH</span><span class="p">(</span><span class="n">bl</span><span class="p">);</span>
<span class="w">    </span><span class="p">}</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">ENCODE_START</span></code> macro writes a header that specifies a <em>version</em> and
a <em>compat_version</em> (both initially 1).  The message version is incremented
whenever a change is made to the encoding.  The compat_version is incremented
only if the change will break existing decoders -- decoders are tolerant
of trailing bytes, so changes that add fields at the end of the structure
do not require incrementing compat_version.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">DECODE_START</span></code> macro takes an argument specifying the most recent
message version that the code can handle.  This is compared with the
compat_version encoded in the message, and if the message is too new then
an exception will be thrown.  Because changes to compat_verison are rare,
this isn’t usually something to worry about when adding fields.</p>
<p>In practice, changes to encoding usually involve simply adding the desired fields
at the end of the <code class="docutils literal notranslate"><span class="pre">encode</span></code> and <code class="docutils literal notranslate"><span class="pre">decode</span></code> functions, and incrementing
the versions in <code class="docutils literal notranslate"><span class="pre">ENCODE_START</span></code> and <code class="docutils literal notranslate"><span class="pre">DECODE_START</span></code>.  For example, here’s how
to add a third field to <code class="docutils literal notranslate"><span class="pre">AcmeClass</span></code>:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">AcmeClass</span>
<span class="p">{</span>
<span class="w">    </span><span class="kt">int</span><span class="w"> </span><span class="n">member1</span><span class="p">;</span>
<span class="w">    </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">member2</span><span class="p">;</span>
<span class="w">    </span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span><span class="w"> </span><span class="n">member3</span><span class="p">;</span>

<span class="w">    </span><span class="kt">void</span><span class="w"> </span><span class="nf">encode</span><span class="p">(</span><span class="n">bufferlist</span><span class="w"> </span><span class="o">&amp;</span><span class="n">bl</span><span class="p">)</span>
<span class="w">    </span><span class="p">{</span>
<span class="w">        </span><span class="n">ENCODE_START</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">encode</span><span class="p">(</span><span class="n">member1</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">encode</span><span class="p">(</span><span class="n">member2</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">encode</span><span class="p">(</span><span class="n">member3</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="n">ENCODE_FINISH</span><span class="p">(</span><span class="n">bl</span><span class="p">);</span>
<span class="w">    </span><span class="p">}</span>

<span class="w">    </span><span class="kt">void</span><span class="w"> </span><span class="nf">decode</span><span class="p">(</span><span class="n">bufferlist</span><span class="o">::</span><span class="n">iterator</span><span class="w"> </span><span class="o">&amp;</span><span class="n">bl</span><span class="p">)</span>
<span class="w">    </span><span class="p">{</span>
<span class="w">        </span><span class="n">DECODE_START</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">decode</span><span class="p">(</span><span class="n">member1</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="o">::</span><span class="n">decode</span><span class="p">(</span><span class="n">member2</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">struct_v</span><span class="w"> </span><span class="o">&gt;=</span><span class="w"> </span><span class="mi">2</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w">            </span><span class="o">::</span><span class="n">decode</span><span class="p">(</span><span class="n">member3</span><span class="p">,</span><span class="w"> </span><span class="n">bl</span><span class="p">);</span>
<span class="w">        </span><span class="p">}</span>
<span class="w">        </span><span class="n">DECODE_FINISH</span><span class="p">(</span><span class="n">bl</span><span class="p">);</span>
<span class="w">    </span><span class="p">}</span>
<span class="p">};</span>
</pre></div>
</div>
<p>Note that the compat_version did not change because the encoded message
will still be decodable by versions of the code that only understand
version 1 -- they will just ignore the trailing bytes where we encode <code class="docutils literal notranslate"><span class="pre">member3</span></code>.</p>
<p>In the <code class="docutils literal notranslate"><span class="pre">decode</span></code> function, decoding the new field is conditional: this is
because we might still be passed older-versioned messages that do not
have the field.  The <code class="docutils literal notranslate"><span class="pre">struct_v</span></code> variable is a local set by the <code class="docutils literal notranslate"><span class="pre">DECODE_START</span></code>
macro.</p>
</section>
</section>



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